The present invention may be employed to control DC motors in a variety of settings. The present invention is particularly effective at controlling DC motors of scale model trains, and that application of the present invention will be described in detail herein. However, in its broadest form, the present invention may be applied in any setting where control of the speed of a DC motor is relatively critical, especially when the motor is connected in parallel with an element, such as the filament of an electric lamp, the resistance of which may vary. Thus, while the following discussion discusses the present invention in terms of model railroading, the scope of the invention is defined in the appended claims and not the following detailed description.
In the hobby of model railroading, the hobbyist attempts to model an entire town and set it into motion. Such towns are replete with buildings, roads, vegetation, and cars all manufactured to scale and painstakingly assembled and decorated to achieve a high level of realism.
Central to these towns is the model railroad system itself. Such systems comprise tracks, gates, switches, bridges, and model locomotives and rail cars. These components of the model railroad system are normally built to the same scale as the surrounding town. Additionally, the hobbyist takes excruciating care to ensure that the physical appearance of these components matches the physical appearance of the full size train upon which the model train is based. Again, the hobbyist's goal is to construct a system that appears realistic in the smallest detail.
In addition to having a realistic appearance, a model locomotive is designed with the goal of operating in a realistic fashion. The scale model locomotive is not per se part of the invention, but will be described herein to the extent necessary for a complete understanding of the present invention.
Basically, a typical scale model locomotive comprises: (a) a main frame, (b) a DC electric motor mounted on the main frame, (c) front and back truck assemblies rotatably mounted onto the main frame, (d) first and second sets of metal wheels rotatably attached to the truck assemblies, (e) a drive transmission for transferring the rotational output of the DC motor to the wheels, and (f) one or more lamps mounted on the main frame.
Depicted in FIG. 1 is a schematic diagram showing the electrical system 2 of a typical model locomotive. As shown in this Figure, a motor 4 is electrically connected in parallel to two lamps 6. The motor 4 comprises an armature 4a that may be represented in an equivalent circuit as a resistance (Ram), an inductance (Larm), and a DC voltage source (Varm). The lamp 6 comprises a filament 6a having a resistance of Rlamp. Indicated schematically at 8a and 8b are first and second sets of wheels respectively.
These locomotives are precision devices and contain little or no space for additional components.
In operation, the locomotive is placed on the tracks, which are metal, so that the wheels contact the tracks. A DC power signal, either voltage or current, is then applied to the tracks. Because the tracks and wheels are metal, current flows through one of the left or right tracks, through the corresponding set of wheels, into the DC motor 4, through the other set of wheels, and through the other of the left or right tracks, thereby controlling the DC motor 4. When the DC motor 4 is energized, an output shaft of the DC motor 4 rotates, which in turn rotates the wheels 8a,b through the drive transmission to move the locomotive. Current also flows through and energizes the filament 6a of the lamp 6 when the motor is energized.
By varying the levels of the DC power on the track, the speed of the locomotive can be varied. Ideally, these levels could be varied to achieve a range of speeds of the model locomotive corresponding to a speed range of 1-100 mph. Normally, the DC power signal is within the range of 0-14 volts.